107. Binary Tree Level Order Tra

因为要求bottom-up 可以考虑使用一个stack。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if(root==null)return result;
        Stack<List<Integer>> stack = new Stack<>();
        ArrayDeque<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        while(!queue.isEmpty())
        {
            int size = queue.size();
            List<Integer>  list = new ArrayList<>();
            for(int i = 0 ;i<size;i++)
            {
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left!=null)
                    queue.add(node.left);
                if(node.right!=null)
                    queue.add(node.right);
            }
            stack.push(list);
        }
        while(!stack.isEmpty())
        {
            result.add(stack.pop());
        }
        return result ;
    }
}